There is a known system as follows. A terminal device acquires a travel history of a vehicle on which it is mounted and then transmits the travel history to a center device via either a short-range wireless communication means used between the terminal device and a road side device or a wide-range wireless communication means such as a portable telephone. A center device collects travel histories of vehicles transmitted from terminal devices, thereby generating information such as traffic information and traffic congestion information based on the collected travel histories.
This type of system known so far includes the following.
Japanese Patent Unexamined Publication No. H11-161894 describes a traffic control system used by the police or other authorities. In this system, sensors are installed in the road to detect traffic information, whereas a center device collects the traffic information and analyzes the travel conditions of vehicles on the road network based on the collected traffic information.
This system, however, can acquire road traffic conditions only on the roads having the sensors installed therein and more specifically only on the points where the sensors are installed.
To overcome this problem, a newer technique is described in Japanese Patent Unexamined Publication No. 2000-311286. In this technique, a vehicle is mounted with a terminal device to acquire a travel history of the vehicle and transmits it to a center device. The center device receives travel histories from terminal devices and analyzes them to generate information such as traffic information and traffic congestion information.
The travel history acquired by a terminal device is transmitted to the center device via either a wide-range wireless communication means such as a portable telephone or a short-range wireless communication using an optical beacon or another road side device.
However, this conventional technique has the following problems. In the case of using a short-range wireless communication means between a vehicle and a road side device such as an optical beacon, a radio beacon, or DSRC, the road side device communicates with one vehicle at a time. This allows the communication network to have a light load, but the center device cannot receive a travel history of a vehicle unless the vehicle passes through the point where the road side device is installed.
In the case of using a wide-range wireless communication means such as a portable telephone or a digital MCA to collect a travel history of a vehicle, as the number of vehicles from which to collect travel histories increases, the communication network has a higher load, causing convergence and making it impossible to use the communication means.
In order to keep the road conditions analyzed by the center device up to date, travel histories of vehicles are required to be collected and updated at regular short intervals. This shortens the road section from which to collect a travel history of each vehicle within the collection interval. As a result, the number of vehicles required to collect the road conditions of the entire road network increases, thereby causing the communication network to have a heavy load.
The load of the communication network can be reduced by accumulating travel histories until a prescribed distance, a prescribed point, or a prescribed time period is reached and then transmitting them all at once to the center device. However, the longer a vehicle accumulates travel histories, the older the travel histories become depending on the travel route. This may cause the information of the congestion section collected by the center device to be too old to be practical.
The time elapsed from the generation of a travel history to the transmission of the travel history to the center device can be reduced as follows. In the case of using road side devices, the number of road side devices installed can be increased. In the case of using a wide-range wireless communication means, the amount of data of travel histories to be accumulated can be reduced so as to increase the frequency of transmitting the travel histories to the center device. However, the former approach requires time and cost to install the road side devices, and the latter approach results in an increase in the communication load on the wide-range wireless communication means.